Fuzzy Self-Adaptive PID Controller for Freeway Ramp Metering

Aiming at the nonlinear and time-varying characteristics of freeway traffic system, a fuzzy self-adaptive PID controller is designed and applied to freeway ramp metering in this paper. A traffic flow model to describe the freeway flow process is firstly built. Based on the model and in conjunction with nonlinear feedback theory, a fuzzy-PID ramp controller is then designed. The ramp metering rate is determined by the PID controller whose parameters are tuned by fuzzy logic according to the density tracking error and error variation. Gauss and triangle curves are used for the membership functions of the fuzzy variables. The rule base including 49 fuzzy rules is also established. Finally, the control system is simulated in MATLAB software. The results show that this controller designed has fast response, good dynamic and steady-state performance. It can achieve a desired traffic density along the mainline of a freeway, and can make vehicles travel more efficiently and safely. This approach is quite effective to freeway ramp metering.